1. Field of the Invention
The present invention relates to an apparatus for irreversibly indicating whether the maximum storage temperature of products stored in the frozen or deep-frozen state has been temporarily exceeded, consisting of a container which has a heat-conducting connection to the stored product and contains an inorganic eutectic mixture or an organic compound which has a melting point in the range from about 0.degree. C. to -50.degree. C., the container containing clearly indicating parts.
In the storage of foods in the frozen state, a storage temperature of at least -16.degree. C. must be maintained if a relatively long shelf life is to be ensured. If, on the other hand, a temperature of -10.degree. to -15.degree. C. is exceeded, the shelf life of many foods is no longer ensured; the frozen material is considered to have thawed although it still appears frozen on the outside.
Usually, freezers are equipped with built-in thermometers which indicate the temperature in a certain part of the freezing space, or a conventional thermometer is placed on or next to the frozen material and is removed from the freezer for readings. In these cases, there is no guarantee that the temperature of the frozen material will be measured exactly. Moreover, air convection in the freezing space may be hindered by stacking and packing of the frozen material, so that the minimum temperatures required for ensuring the shelf life of the foods may be exceeded in certain parts of the freezing space, even when the equipment functions satisfactorily. The user of the frozen material cannot detect the fact that the maximum storage temperature has been exceeded, so that he cannot counteract spoilage of the food by removing it in good time and using it immediately. The danger that deep-frozen food will be thawed without this being noticed is particularly great, for example, when a relatively large amount of material to be chilled is introduced in a warm state into the freezer.
It is therefore desirable to have a temperature indicator which shows, for every food pack or for a group of packs, whether they have been sufficiently chilled during their entire storage time.
2. Description of the Related Art
Temperature-indicating apparatuses of the generic type stated at the outset are disclosed, for example, in DE 25 47 638 and 32 43 031. In the first-mentioned Laid-Open Application, the indicator element is a contact layer of absorptive material, over which a layer of a dye soluble in a liberated liquid is arranged, over which layer in turn an indicating layer of absorptive material is present. When the frozen material partially thaws, cellular liquid is released at its surface and is absorbed by the contact layer. The liquid thus absorbed dissolves part of the colour layer and the coloured liquid is then absorbed by the indicator layer whose discoloration provides evidence of reduction in quality due to interruption of the cooling chain.
The last-mentioned application describes a container having transparent walls, for example a flat bag, which contains a eutectic mixture, those walls of the container which face the frozen material bearing a recognizable inscription on their inner surface. When the temperature falls below the critical temperature, the inscription is invisible; it become visible only when the critical temperature is exceeded. This publication also states that an absorptive material can be placed between the frozen material and the container described, which absorptive material absorbs the liquid taken up on exceeding the critical temperature and in this way detects when the permissible temperature has been exceeded.
The two abovementioned temperature indicators have a complicated design; moreover, it is not ensured that the frozen product does not come into contact with the indicator medium and thus become damaged.